Thermionic electron emitters are devices in which electrons are thermionically emitted from the surface of a material upon the application of heat energy. Thermionic electron emitters can be used to provide a current, as electron beam sources (for example, in televisions or computer monitors), microwave generators, thermionic generators, vacuum diode heat pumps, amplifiers for broadcasting, electron microscopes and electron sources employed in propulsion systems, for example, as are used in spacecraft propulsion systems.
For example, thermionic energy conversion is a technique in which heat energy is converted to electrical energy by thermionic emission. Electrons are thermionically emitted from the surface of a material, such as a metal, by heating the metal. Sufficient energy is imparted to a portion of the electrons to overcome retarding forces at the surface of the metal so that these electrons are emitted at the surface of the metal. In contrast to many other techniques of generating electrical energy, thermionic conversion typically does not require an intermediate form of energy or a working fluid to convert heat into electricity.
Thermionic energy converters typically include an emitter electrode connected to a heat source and a collector electrode connected to a heat sink. The electrodes are separated by a space or gap, and leads connect the electrodes to an electrical load. The space between the electrodes is typically either under vacuum or filled with a suitable vapor. The heat source supplies heat to raise the temperature of the emitter electrode to a sufficiently high temperature so that the electrons are thermionically emitted into the gap and then onto the collector electrode. The electrons are captured at the collector electrode and return to the emitter electrode via the leads and the electrical load between the emitter and the collector.
The emitter electrode or cathode typically has a relatively low electron work function to allow emission of the electrodes. The performance of thermionic energy converters may be limited by the work function of the materials from which the emitters are made and the space charge effect. In other words, the presence of charged electrons in the space between the emitter and the collector can create an extra potential barrier that reduces the thermionic current.
For example, thermionic energy converters typically utilize planar metal based emitters, which may result in high operating temperatures as well as performance limitations due to space charge effects. The high operation temperature of these systems may limit their applications.